1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting/receiving data in a communication system using multiple antennas (hereinafter referred to as a “multi-antenna communication system”), and in particular, to an apparatus and method for spatial multiplexing transmission in a multi-antenna communication system.
2. Description of the Related Art
A wireless communication system has been developed to allow users to perform communication without distance limitations. A mobile communication system is the typical wireless communication system. The mobile communication system has evolved from the early voice service communication system into a high-speed, high-quality wireless packet data communication system for providing data service and multimedia service. It can be considered that the ongoing standardization on High Speed Downlink Packet Access (HSDPA) led by 3rd Generation Partnership Project (3GPP), and Evolution for Data and Voice (EV-DV) and Evolution-Data Only (EV-DO) led by 3rd Generation Partnership Project 2 (3GPP2) is a typical attempt to find a solution for the high-speed, high quality wireless packet data transmission service at 2 Mbps or higher in the 3rd generation (3G) mobile communication system.
Meanwhile, research on the 4th generation (4G) mobile communication system is being conducted with the aim of providing higher-speed, higher-quality multimedia service using Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA).
The current 3G wireless packet data communication systems, like the HSDPA, EV-DV and EV-DO, use an Adaptive Modulation and Coding (AMC) method and a channel sensitive scheduling resource management method to improve transmission efficiency. With the use of the AMC method, a transmitter can adjust the amount of transmission data according to channel status. That is, the transmitter decreases the amount of transmission data in a channel having bad status and increases the amount of transmission data in a channel having good status, thereby efficiently transmitting the large amount of data while maintaining a desired reception error probability.
With the channel sensitive scheduling resource management method, a transmitter selects a good-channel status user, thereby increasing the data throughput. In the AMC method and the channel sensitive scheduling resource management method, the transmitter receives partial channel status information fed back from a receiver, and applies an appropriate modulation and coding technique determined at the time to be most efficient.
In order for the AMC method and the channel sensitive scheduling resource management method to help improve the system capacity substantially, the channel status information fed back by the receiver should be matched to the channel status at the transmission time. Generally, however, in the mobile communication environment, because the transmitter or the receiver moves from place to place, the channel status changes continuously. The continuous change in the channel status is related to moving velocity of the transmitter or the receiver, and this is called Doppler spread. High moving velocity deepens the Doppler spread. In this case, the channel status information fed back by the receiver may be invalid.
In this situation, therefore, even the use of the AMC method and the channel sensitive scheduling resource management method cannot accomplish system capacity improvement. To make up for the defects, the 3G wireless packet data communication system adopts Hybrid Automatic ReQuest (HARQ). In the HARQ technology, when a receiver fails to normally receive the data transmitted by a transmitter, the receiver immediately informs the transmitter of the failure so that the transmitter rapidly performs retransmission in a physical layer.
Meanwhile, an OFDM scheme, recently spotlighted in the wireless communication system, sends modulation signals on orthogonal frequency signals, that is, sub-carriers. Therefore, an OFDMA scheme is a method for sending different user signals on different sub-carriers based on the OFDM scheme. In the OFDMA scheme, channel sensitive scheduling which could be performed only in the time axis can be performed even in the frequency axis. That is, for data transmission, the OFDMA system schedules a sub-carrier preferred by each user in the frequency selective fading environment through frequency scheduling, thereby improving the system capacity as compared with the case where scheduling is performed only in the time axis. Therefore, in order to efficiently perform frequency scheduling, it is preferable to use a bundle of adjacent sub-carriers having a similar channel response for data transmission taking into account of overhead of the channel status feedback.
Further, a Multiple-Input Multiple-Output (MIMO) system using multiple antennas for a transmitter and a receiver is now under active discussion as a core technology for providing high-speed, high-quality data service in wireless communication. Theoretically, it is known in the MIMO system that as the number of transmission/reception antennas increases without an additional increase in frequency bandwidth, the serviceable data capacity linearly increases in proportion to the number of transmission/reception antennas. Therefore, the MIMO-based technology can be roughly divided into a Spatial Diversity technique and a Spatial Domain Multiplexing (SDM) technique according to its purpose. A description will now be made of the Spatial Diversity technique and the SDM technique.
The Spatial Diversity technique is developed to prevent a reduction in link performance due to fading occurring in a mobile communication channel by using multiple transmission/reception antennas. The Spatial Diversity technique can efficiently reduce a reception error probability, when a transmitter has channel status information and cannot adaptively adjust the amount of transmission information. The SDM technique is developed to transmit the larger amount of data using the MIMO scheme, as compared with the single-transmission/reception antenna technique. The SDM technique can efficiently increase data throughput in an environment where the spatial correlation is very low due to the scattering objects having a sufficient number of channel environments.
The SDM technique extended based on multiple access is a Spatial Domain Multiple Access (SDMA) technique. The SDM technique increases the number of transmission channels over which data is transmitted, using multiple transmission/reception antennas. In an environment where the spatial correlation is low, it is possible to increase the data throughput using the SDM technique. However, in an environment where the spatial correlation is high, even though the data throughput is increased, it is not possible to prevent an increase in the reception error probability. However, in an environment where the spatial correlation is high, if transmission channels over which the data (the amount of which is increased with the use of the MIMO scheme) is transmitted are allocated to different users, it helps increase the system capacity. This is because if the spatial correlation is high, interference between user signals having different spatial characteristics may decrease. That is, the SDMA technique is a spatial processing technique capable of increasing the system capacity in the high-spatial correlation environment.
Each of such spatial processing techniques as Spatial Diversity technique, SDM technique and SDMA technique differs in available scope according to traffic type and channel environment showing capacity improvement. For example, for a voice call, because similar amounts of data are always generated, it is difficult to apply the AMC method and the channel sensitive scheduling resource management method, both of which vary the data throughput. In addition, if the channel status becomes bad due to fading, a reception error is unavoidable. In this situation, the use of the Spatial Diversity technique can prevent the channel status from deteriorating. In the channel environment where the spatial correlation is low, the use of the SDM technique can increase the data throughput, together with the AMC method and the channel sensitive scheduling resource management method. In the channel environment where the spatial correlation is high, the SDMA technique can improve the system capacity. Therefore, there is a need to appropriately select the spatial processing technique according to the channel environment and traffic type.
A description will now be made of a transmitter for each of the systems described above.
FIG. 1 is a diagram illustrating a structure of a transmitter in a wireless communication system employing Space-Time Coding (STC). With reference to FIG. 1, a description will now be made of an apparatus and method for transmitting data using STC coding.
If data (such as an information bit stream) 10 that an upper layer desires to transmit is received, the data 10 is input to an AMC unit 100. The AMC unit 100 includes therein a channel coding/modulation unit 110 and an AMC controller 101. The channel coding/modulation unit 110 includes therein a channel encoder 111, a channel interleaver 112, and a modulator 113. Therefore, the data 10 is input to the channel encoder 111. The data coded by the channel encoder 111 is dispersed (or permutated) by the channel interleaver 112. The reason for dispersing the data by the channel interleaver 112 is to prevent deterioration of coding performance due to fading during data transmission. The data dispersed by the channel interleaver 112 is converted into a modulation signal by the modulator 113. A series of processes where the data undergoes coding 111, interleaving 112, and modulation 113 is called a “channel coding and modulation” process. Therefore, this process is performed in the channel coding/modulation unit 110.
The channel coding/modulation unit 110 can apply a different scheme depending on Channel Status Information (CSI) feedback 105 that a receiver has delivered according to the system. For example, if the channel status is good, the channel coding/modulation unit 110 increases a channel coding rate and a modulation order so that the increased amount of data is transmitted. However, if the channel status is bad, the channel coding/modulation unit 110 decreases the coding rate and the modulation order so that the decreased amount of data is transmitted more reliably. In this way, the transmitter applies AMC based on the CSI feedback 105, and the AMC controller 101 determines which coding and modulation schemes it will use. The AMC controller may be omitted, according to the particular design. That is, it is meant in FIG. 1 that the signals shown by a dotted line are optional. For example, in the case where several users receive the same information such as a broadcast, because it is not possible to adaptively change the coding and modulation schemes according to a channel status of a particular user, the transmitter need not support AMC. A process in which the AMC controller 101 adaptively changes the channel coding and modulation schemes based on the CSI feedback 105 is called an AMC process. Therefore, an apparatus provided for performing the AMC process is shown as an AMC unit 100 in FIG. 1.
The signal modulated by the AMC unit 100 is STC-coded by an STC encoder 121. The STC encoder 121 typically adopts an Alamouti coding method, which is applied to two transmission antennas. The Alamouti coding method, which corresponds to Orthogonal Space Time Coding (OSTC), can obtain the maximum diversity gain. In the Alamouti coding method, there should be no change in the channel between Alamouti-coded adjacent time signals in order to maintain orthogonality. If the channel between the adjacent time signals undergoes an abrupt change, the Alamouti coding method cannot guarantee orthogonality, causing self-interference and performance deterioration due thereto.
However, it is known that the OSTC coding method guaranteeing orthogonality provides the maximum diversity gain. The general STC coding method is performed without depending on the CSI that the receiver feeds back. Therefore, the STC coding method is designed to maximize the diversity gain, rather than being modified to be adaptive to the channel. The signal obtained by STC-coding the modulation signal is converted into a transmission band signal by a Radio Frequency (RF) unit 122, generating a plurality of symbols to be transmitted via multiple transmission antennas 131 through 132. For example, the STC coding method applied to a 2-transmission antenna system receives one data stream and outputs two symbol streams. The generated symbol streams are transmitted via different transmission antennas 131 and 132.
The symbol stream to which the STC coding method is applied is converted by the RF unit 122 into an RF signal to be transmitted via a transmission antenna. The RF unit 122 performs filtering for satisfying a spectrum characteristic, adjusts transmission power, and converts a baseband signal into an RF signal. After this process, the output signals are transmitted via their associated antennas 131 and 132.
FIG. 2 is a diagram illustrating a structure of a transmitter employing OFDM-STC coding. With reference to FIG. 2, a description will now be made of a structure and operation of a transmitter employing OFDM-STC coding.
An AMC unit 100 is equal to that of FIG. 1 in operation, so a description thereof is omitted here. The signal modulated by the AMC unit 100 is input to an OFDM modulator 210. The OFDM modulator 210 includes therein an Inverse Fast Fourier Transform (IFFT) unit 211 and a Cyclic Prefix (CP) symbol adder 212. Therefore, the signal modulated by the AMC unit 100 is input to the IFFT unit 211. The IFFT unit 211 performs IFFT on the modulation signal such that the modulated signals should be carried on orthogonal frequency signals, that is, sub-carriers. Thereafter, a CP symbol is added to the IFFT-processed signal by the CP symbol adder 212. The CP is achieved by copying a part of the last part of a generated sub-carrier and adding the copied part to the head of the symbol, to maintain the orthogonality between sub-carriers even though delay spread occurs due to multi-path fading, thereby preventing interference. If OFDM symbols are generated by the OFDM modulator 210, an STC encoder 121 performs STC coding on consecutive OFDM symbols. Thereafter, an RF unit 122 converts the STC-coded OFDM symbols into transmission band RF signals, and then transmits the RF signals via multiple transmission antennas 131 and 132. In the transmitter employing OFDM-STC coding, if a change in the channel between consecutive OFDM symbols is considerable, the orthogonality may be affected, causing interference.
FIG. 3 is a block diagram illustrating a structure of a transmitter employing OFDM-SFC coding. A description will now be made of a structure and operation of a transmitter employing OFDM-SFC (Space Frequency Coding) coding.
In the OFDM scheme, modulation signals can be carried on different frequencies at different times. Therefore, an SFC coding method can be implemented by applying the STC coding method to consecutive frequency signals, that is, sub-carriers, instead of applying the STC coding method to consecutive time signals. An AMC unit 100 outputs a modulated signal using an input information bit stream 10 and SCI feedback information 105. The output modulation signal is input to an SFC encoder 300. The SFC encoder 300 performs the same process as the STC coding process, but finally applies STC coding to the consecutive frequency signals, so this coding method is called an SFC coding method. The signal stream modulated by the SFC encoder 300 is obtained by applying STC coding to consecutive time signals. One stream is coded into a plurality of streams by the SFC encoder 300, and the streams are separately modulated by ODFM modulators 210a to 210n. The STC-coded signals are converted into OFDM signals by the OFDM modulators 210a to 210n. It can be considered herein that the STC coding applied to the consecutive time signals by the OFDM modulators 210a to 210n are applied to consecutive frequency signals. Thereafter, the OFDM signals are converted into transmission signals by an RF unit 122, and then transmitted via associated transmission antennas 131 and 132. In the transmitter employing OFDM-SFC coding, if a change in the channel between consecutive sub-carriers is considerable, the orthogonality may be affected, causing interference.
FIGS. 2 and 3 respectively show a transmission scheme employing STC coding, and a transmission scheme employing SFC coding that implements STC coding in the frequency axis. Such conventional techniques were designed to obtain the maximum spatial diversity gain, as described above. Therefore, such techniques show excellent link performance in the low-spatial correlation environment, but cannot provide a multi-antenna gain if the spatial correlation increases. The reason for showing the excellent link performance in the low-spatial correlation environment is because the use of the diversity technique, like other kinds of diversity techniques, can reduce the change in channel with the passage of time. However, the diversity technique helps reduce the probability of channel deterioration by reducing the change in the channel, but reduces a large number of channels capable of transmitting data. For this reason, it is known that in the system supporting AMC and channel sensitive scheduling, the diversity technique reduces the system capacity undesirably.
FIG. 4 is a block diagram illustrating a structure of a transmitter employing OFDM-based spatial multiplexing. With reference to FIG. 4, a description will now be made of a structure and operation of a transmitter employing OFDM-based spatial multiplexing.
Before a description of FIG. 4 is given, the foregoing techniques and a spatial multiplexing technique will be described in brief. With the use of multiple transmission antennas, the STC coding technique and the SFC coding technique transmit one data stream, whereas the spatial multiplexing technique transmits a plurality of data streams. In the channel environment where the low-spatial correlation MIMO scheme is used because of a large number of scattering objects, a predetermined number of data streams can be transmitted, wherein the predetermined number corresponds to a smaller one from among the number of transmission antennas and the number of reception antennas. For example, in the above channel environment, if the number of transmission antennas is 2 and the number of reception antennas is 4, the transmitter can transmit 2 data streams. Therefore, in order for the system of FIG. 4 to stably operate, the receiver also needs more than M antennas. For reference, because the STC coding technique and the SFC coding scheme transmit only one data stream, they do not need a plurality of reception antennas.
In the MIMO system, it is known that in order to increase the capacity, it is preferable to increase the number of transmission data streams rather than improving a Signal-to-Noise Ratio (SNR) with the number of transmission data streams being fixed to one. Therefore, the spatial multiplexing technique uses such characteristics of the MIMO system.
Referring to FIG. 4, transmission data streams are individually input to AMC units 100a to 100n where they undergo an AMC process independently. Thereafter, the resulting streams are OFDM-modulated by OFDM modulators 210a to 210n. The OFDM-modulated symbols are converted into RF transmission signals by an RF unit 122, and then transmitted to a receiver via multiple transmission antennas 131 and 132. That is, different data streams are transmitted via different transmission antennas. The AMC units 100a to loon shown in FIG. 4 can be used when the system performs CSI feedback. If the CSI is not fed back, the fixed coding and modulation schemes are performed. In this case, the AMC units 100a to 100n perform the fixed coding and modulation schemes.
In the spatial multiplexing technique, there are two methods of performing AMC using CSI feedback. A first method applies the same AMC method for all transmission antennas. In order to support this method, a receiver is allowed to feed back only one representative CSI. A second method applies different AMC methods for all transmission antennas. In order to support this method, the receiver should feed back a CSI corresponding to each of the transmission antennas. That is, the former method is less than the latter method in terms of the CSI feedback overhead. However, in the former method, because only one AMC is applied to different transmission antennas experiencing different channel statuses, the capacity improvement effect of the system supporting AMC and channel sensitive scheduling is reduced. FIG. 4 shows the latter method that allows the receiver to feed back a CSI for each of the transmission antennas, and shows the transmitter employing an AMC method according thereto. Such a spatial multiplexing method is known as Per Antenna Rate Control (PARC).
FIG. 5 is a block diagram illustrating a structure of a transmitter employing PARC in a system supporting AMC and channel sensitive scheduling. With reference to FIG. 5, a description will now be made of a structure and operation of a transmitter employing PARC in a system supporting AMC and channel sensitive scheduling.
A scheduler 501 receives transmission data 10a to 10n of K multiple users from an upper layer. Herein, the scheduler 501 is a channel sensitive scheduler (hereinafter referred to as “scheduler” for simplicity). The scheduler 501 selects the most preferred user terminal to which it will transmit data at the present time, based on the CSI fed back from each user terminal. In order to deliver the necessary control information in the following process of transmitting data of the selected user, the scheduler 501 provides scheduling information 510 to an AMC controller 505. Then the AMC controller 505 analyzes the scheduled user and issues an AMC command according to a channel status of the user. That is, the scheduler 501 generates the information indicating via which antenna and with which coding and modulation methods it will transmit transmission data, and provides the generated information to the AMC controller 505. Therefore, the AMC controller 505 can determine the number of transmission data streams and a size of the transmission data for each individual antenna, based on the information on the transmission antennas.
A multiplexer 503 multiplexes user information bit stream scheduled by the scheduler 501 according to the number of transmission antennas and a data rate for each individual antenna based on the information provided from the AMC controller 505. For example, the multiplexer 503 multiplexes the scheduled user's information bit stream such that the larger amount of data stream should be transmitted via the transmission antenna having a good channel status. In the next process, PARC is applied to the multiplexed data stream. That is, the multiplexed data streams are individually input to channel coding/modulation units 110a to 110m where they are coded and modulated. The coded/modulated data streams are individually input to OFDM modulators 210a to 210m where they are OFDM-modulated. Thereafter, the OFDM-modulated data streams are converted into RF signals, and transmitted via associated antennas 131 to 132.
The system shown in FIG. 5 selects only one user at the transmission time and transmits data streams over the full band. That is, the system is not an OFDMA system. However, it is possible to simply extend the system to an OFDMA system supporting AMC and channel sensitive scheduling by dividing the full system band into sub-channels each composed of adjacent sub-carriers and independently applying PARC to each of the sub-channels. However, because only one user signal is transmitted via a plurality of antennas, SDMA is not implemented.
The two different spatial processing techniques have been described as the conventional MIMO technology.
The first technique, a spatial diversity technique, fixes the number of transmission data streams to one to reduce the change in the channel with the passage of time. The second technique, a spatial multiplexing technique, transmits a plurality of data streams.
The STC coding and SFC coding techniques described in FIGS. 2 and 3 can be classified as the spatial diversity technique. As described above, the diversity technique helps reduce the probability of channel deterioration by reducing the change in the channel, but reduces a large number of channels capable of transmitting data. For this reason, it is known that the spatial diversity technique reduces the system capacity undesirably. However, the spatial diversity helps broaden the coverage for the traffic that the transmitter can hardly change the transmission method according to the channel status, like the broadcast.
Although the STC coding technique and the SFC coding technique were designed to maintain orthogonality in order to obtain the maximum spatial diversity gain, if the adjacent channels change, the techniques cause self-interference undesirably. For example, the STC coding technique helps complement the AMC and channel sensitive scheduling in the fast-moving environment, but decreases in link performance undesirably because the orthogonality is damaged. In the case of the SFC coding technique, because adjacent sub-carriers experience different channel responses in the environment where the time delay spread is considerable, the link performance deteriorates even in this case due to the affect on orthogonality.
The PARC technique described in FIGS. 4 and 5 can be classified as a spatial multiplexing technique. The PARC technique has disadvantages in that reception performance deteriorates in the high-spatial correlation environment. Because the PARC technique performs AMC by feeding back only the channel status of each individual transmission channel, it undesirably transmits the data whose amount exceeds the capacity supportable by the channel in the high-spatial correlation environment. The high spatial correlation means that there is a high probability that when one transmission antenna experiences a good channel status, other transmission antennas will also experience the good channel status. However, because the channels experienced by different antennas are similar to each other, the receiver cannot separate the signals transmitted from the different antennas. Therefore, interference occurs between the simultaneously transmitted streams, deteriorating the reception link performance. The PARC technique cannot avoid the deterioration of the reception link performance due to spatial correlation because it was designed on the assumption that the channels have no spatial correlation.
As another problem, the PARC technique does not support SDMA. The system supporting AMC and channel sensitive scheduling improves the system capacity by obtaining a multi-user diversity gain. The multi-user diversity selects a proper user through scheduling in the channel-varying mobile communication environment and transmits data to the selected user. The system capacity decreases undesirably as compared with the environment where the channels do not vary. Accordingly, there is a need for an improved apparatus and method for transmitting/receiving data in a multi-antenna communication system.